fix(sigv4): Makes chunking opt-in

[dnys1]

This was made the default since it's much more efficient, especially on web for large uploads, but some S3 operations do not support it so it should not be the default. Instead, it will be selectively enabled in the SDK layer for operations which allow it.

Also cleans up example.

[codecov-commenter]

Codecov Report

Merging #1820 (a4f9ccf) into main (6a5fdfc) will increase coverage by 0.32%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1820      +/-   ##
==========================================
+ Coverage   42.14%   42.47%   +0.32%     
==========================================
  Files         371      376       +5     
  Lines       10768    10873     +105     
==========================================
+ Hits         4538     4618      +80     
- Misses       6230     6255      +25     

Flag	Coverage Δ
android-unit-tests	∅ <ø> (∅)
flutter-unit-tests	31.45% <ø> (ø)
ios-unit-tests	88.92% <ø> (-0.68%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
.../ios/unit_tests/amplify_flutter_exampleTests.swift	94.59% <0.00%> (ø)
..._pinpoint_ios/example/ios/Runner/AppDelegate.swift	0.00% <0.00%> (ø)
...y_flutter_ios/example/ios/Runner/AppDelegate.swift	0.00% <0.00%> (ø)
...e/ios/unit_tests/MockAnalyticsCategoryPlugin.swift	15.38% <0.00%> (ø)
...ios/example/ios/unit_tests/AtomicResultTests.swift	87.75% <0.00%> (ø)